Pultrusion is a known technique in which longitudinally continuous fibrous elements, which can include reinforcing fiber and/or a mat, are combined into a resin-based structure. The process generally involves pulling reinforcing fibers and/or reinforcing mats through a bath of thermoset resin and then into a heated forming die. The heat of the die cures the resin as the part is pulled through the die on a continuous basis.
The mat and reinforcing fiber are typically flexible and conformable textile products since they need to conform to the profile of the die. The mat and reinforcing fiber are typically glass products, while the resin matrix is usually, but not necessarily, a thermosetting polyester. Mat material is generally in the form of a non-woven, felt-like web having glass fibers randomly placed in a planar swirl pattern.
During the pultrusion process, reinforcing fibers typically referred to as rovings comprise groupings of hundreds or thousands of microns-diameter filaments, that mechanically behave like flexible rope. The filaments are flexible because the diameter of each filament is so small. The flexibility of the individual filaments imparts sufficient flexibility to the reinforcing fibers to fulfill the processing requirements of pultrusion. In a pultrusion profile, the mat and rovings constitute the reinforcement, while the resin constitutes the binder of the solid composite. After pultrusion, the rovings are held together by the cured or semicured resin matrix, providing the pultruded part with rigidity.
The longitudinal strength of pultruded parts is very high since the majority of the fibers are the longitudinally extending reinforcing fibers that are pulled through the die. However, the transverse strength of pultruded parts is generally minimal because conventional mat fibers extend in random directions and only a small proportion of the total fiber component extends in the transverse direction.
Conventional mats also have a number of problems that interfere with the efficiencies of the pultrusion process. First, the mat is relatively expensive. Second, the mat is difficult to form into the required shape for complex parts. The compressed thickness of the mat also represents a lower limit on the thickness of sidewalls, increasing the amount of resin needed for a given part. Lightweight continuous filament or “swirl” mats are easier to shape, but provide minimal strength, and are more prone to ripping at the die entrance due to low wet tensile strength. The choice of mat is, in part, a compromise between the necessity for bending to shape, the required strength of the pultruded part, and the pulling strength of the reinforcing mat.
U.S. Pat. No. 5,005,242 (Vane) reports a reinforcing mat having a plurality of superimposed layers. Each layer consists of a plurality of uni-directional non-woven yarns or threads laid side-by-side. The yarns in at least some of the different layers extend in different directions. The layers of reinforcing material are stitched together by knitting so as to hold the yarns in fixed position relative to one another. The mat disclosed in Vane exhibits strength primarily in the direction of the unidirectional yarns.
U.S. Pat. No. 5,908,689 (Dana et al.) reports a mat adapted to reinforce a thermosetting matrix material. The mat includes a primary, supporting layer having a plurality of randomly oriented essentially continuous glass fiber strands. The primary layer is about 1 to about 20 weight percent of the mat on a total solids basis. A secondary layer is positioned upon and supported by a surface of the primary layer. The secondary layer includes a plurality of glass fiber strands having a mean average length of about 20 to about 125 millimeters. The strands of the primary layer are entangled with the strands of the secondary layer by needling the primary layer and the secondary layer together.
U.S. Pat. No. 5,910,458 (Beer et al.) reports a mat adapted to reinforce a thermosetting matrix material. The mat includes a primary layer of generally parallel, essentially continuous glass fiber strands oriented generally parallel to a longitudinal axis of the mat. The primary layer is about 45 to about 90 weight percent of the mat on a total solids basis. A secondary layer includes a plurality of randomly oriented, generally continuous glass fiber strands. The strands of the primary layer are entangled with the strands of the secondary layer by needling.
U.S. Pat. No. 4,058,581 (Park) reports adding discontinuous fibers to the resin bath. Similarly, U.S. Pat. No. 5,324,377 (Davies) reports mixing cut fibers in the resin bath to form a homogeneous mass of resin and fibers. The continuous fibers, the cut fibers and the resin are then passed through a die and become integrated into a pultruded part.
In order for the reinforcing mat to pass through the die with the longitudinal fibers, it is necessary for the mat to have a sufficient longitudinal strength so that it does not tear as it is pulled through the die. Furthermore, the mat must have a sufficient shear strength so that it does not twist or skew allowing one side edge of the mat to move in advance of the other side edge. If such twisting or skewing occurs, the mat will become distorted in the part and the mat eventually will break down and the part will be unusable.